Oracle Database Writing Time Period

In Oracle Database environments, the Database Writer process, commonly referred to as DBWn (where 'n' represents the process number starting from 0), plays a crucial role in writing data from the database buffer cache to the datafiles on disk. The DBW0 process, being one of these writer processes, is triggered to write data in several key situations to ensure data integrity, optimize performance, and manage system resources effectively. The primary circumstances that trigger the DBW0 process to write data include:

1. Checkpoint Events: Checkpoints are significant events in an Oracle Database where the database ensures that all dirty buffers (modified data in the buffer cache not yet written to disk) are written to the datafiles. Checkpoints occur at specific intervals, during database shutdown, and in other situations demanding data consistency.
2. Redo Log Switch: When the current online redo log file is filled and Oracle switches to the next available redo log file, a log switch occurs. This event prompts the DBW0 process to write all dirty buffers associated with the redo entries in the filled log to ensure recovery consistency.
3. Space Management: When space in the buffer cache becomes scarce, and there are not enough free buffers for incoming data, the DBW0 process is triggered to write dirty buffers to disk, freeing up space in the buffer cache for new or incoming data.
4. Timeouts: The DBW0 process operates not only based on specific events but also on a timed basis. If certain buffers have remained dirty (modified but not written to disk) for a predetermined amount of time, the DBW0 process will write these buffers to disk to minimize potential data loss in case of an unexpected shutdown.
5. Manual Intervention: Database administrators can manually trigger the DBW0 process through specific administrative commands or operations, such as issuing a manual checkpoint or adjusting the buffer cache's write thresholds.
6. Fast Commit Mechanism: In scenarios of fast commits where transactions commit changes that are still in memory, the DBW0 process may be prompted to write these changes to disk to ensure that the commit operation is durable and the data integrity is maintained.
7. Buffer Cache Pressure: When the buffer cache experiences high demand and contention, the DBW0 process might be triggered more frequently to write out dirty buffers to disk, reducing contention and improving overall database performance.

Understanding these triggers is crucial for Oracle Database Administrators (DBAs) to optimize database performance, plan for capacity, and ensure data integrity. DBAs can influence the behavior of the DBW0 process through various settings and configurations, such as the initialization parameters `DB_WRITER_PROCESSES`, `DB_BLOCK_MAX_DIRTY_TARGET`, and others, to tailor the database's behavior to the specific needs of their environment.

Identify the situations that trigger the DBW0 process to write data.
Three situations will cause the database writer to write dirty buffers back to disk:

1. Insufficient buffer space for data that needs to be read
2. Advancement of the checkpoint
3. The number of dirty blocks exceeds a predefined target

The next three lessons talk about each of these triggering situations in more detail.

The database writer process (DBWn) writes the contents of database buffers to data files. DBWn processes write modified buffers in the database buffer cache to disk. Although one database writer process (DBW0) is adequate for most systems, you can configure additional processes (DBW1 through DBW9 and DBWa through DBWj) to improve write performance if your system modifies data heavily. These additional DBWn processes are not useful on uniprocessor systems. The DBWn process writes dirty buffers to disk under the following conditions:

1. When a server process cannot find a clean reusable buffer after scanning a threshold number of buffers, it signals DBWn to write. DBWn writes dirty buffers to disk asynchronously if possible while performing other processing.
2. DBWn periodically writes buffers to advance the checkpoint, which is the position in the redo thread from which instance recovery begins. The log position of the checkpoint is determined by the oldest dirty buffer in the buffer cache.

In many cases the blocks that DBWn writes are scattered throughout the disk. Thus, the writes tend to be slower than the sequential writes performed by LGWR. DBWn performs multiblock writes when possible to improve efficiency. The number of blocks written in a multiblock write varies by operating system.

DBWR - Database Writer or Dirty Buffer Writer process is responsible for writing dirty buffers from the database block cache to the database data files. Generally, DBWR only writes blocks back to the data files on commit, or when the cache is full and space has to be made for more blocks. The possible multiple DBWR processes in RAC must be coordinated through the locking and global cache processes to ensure efficient processing is accomplished.

Click the Quiz link below to test your knowledge of the database writer.
Database Writer - Quiz